The present invention relates generally to a method and apparatus for applying a plastic material to a transparent sheet, e.g., glass, and, particularly, to a method and apparatus for producing a unique retention shield window assembly which can be used, for example, as a backlight in automotive vehicles.
In the early stages of the automobile industry, single sheets of ordinary glass were employed as windshields, side windows and backlights. As it became evident that this type of glass utilized as a windshield presented a considerable safety hazard, the single sheets of ordinary glass were replaced with single sheets of heat treated or tempered glass. Thereafter, as laminated safety glass was developed to reduce the severity of lacerative injuries, its use in automotive windshields greatly increased until today, when almost all automotive windshields are constructed of some type of laminated glass.
Typically, laminated glass of the type utilized in vehicle windshields consists of two sheets of glass bonded together with a thin plastic interlayer, such as a sheet of polyvinyl butyral for example. In the event of an impact on a laminated glass windshield sufficient to break glass, the plastic interlayer functions to bind the glass fragments together, thus reducing the risk of injury of a driver or passenger as a result of flying glass or contact with the windshield. Further developments of this type of laminated glass, such as those disclosed in U.S. Pat. No. 3,231,461, have resulted in laminated windshields with improved penetration resistance.
As a result of the evergrowing recognition of the necessity for increased safety precautions, continuing efforts have been and are still being made to appreciably reduce the injury producing potential of automobile windows. Side windows and backlights typically have been formed of a single sheet of tempered glass. As the automobile stylists seek to increase the glass area in vehicles, backlights are becoming more complex in shape and are extending over the passenger area. Thus, it has become more important to increase the penetration resistance of backlights to objects impacting the backlight from either the outside or the inside.
Recently, it has been found that the addition of a second plastic layer bonded to the inner glass surface of the laminated windshield further increases the safety effectiveness of the windshield. An example of an automotive windshield which incorporates, as part of its laminated structure, a protective laceration shield bonded to its inboard glass surface is disclosed in U.S. Pat. No. 4,242,403. In this patent, the laceration shield includes a penetration resisting multi-layer body consisting of an inner layer of relatively soft, extensible plastic material such as polyvinyl butyral, for example, which is adhered to the inboard surface of the windshield, an intermediate layer of more durable plastic such as polyester, and an outer coating of an abrasion resistant material.
One method for applying a plastic layer to one surface of a single sheet of glass is disclosed in U.S. Pat. No. 3,806,387. In this method, a sheet of glass, a layer of adhesive, and a layer of thin transparent plastic sheeting are assembled in a stack to produce a laminated assembly. A second sheet of glass (called a glass cover or forming sheet) conforming to the configuration of the sheet of glass in the laminated assembly is then placed on top of the plastic sheet. The surface of the glass forming sheet which is placed adjacent the plastic sheet is coated with a demolding agent to prevent any adhesion between the glass forming sheet and the plastic sheet. Typically the glass forming sheet is made on the same form as the glass sheet of the laminated assembly. Next, the space between the individual laminae are evacuated and the laminated assembly is positioned in an autoclave. The autoclave applies pressure to the exterior surfaces of the laminated assembly while heating the assembly to a temperature which causes bonding between the glass sheet and the plastic sheet. After the assembly is removed from the autoclave, the forming sheet can be removed from the stack.